RT is a cornerstone in treating residual breast cancer postoperatively and can significantly improve long-term survival [33]. Meta-analyses by the Early Breast Cancer Trialists’ Collaborative Group demonstrated prolonged survival benefits, even beyond 15 years, following postoperative radiotherapy. However, radiotherapy induces early and late complications in adjacent tissues, such as the heart and lungs [34].
Radiation-related heart disease (RRHD) is a major concern associated with RT. Thus, reducing heart dose is crucial for minimizing RRHD. This analysis revealed that DIBH yielded significantly lower heart Dmean than FB. Heart V5, V10, V20, V25, and V30 also demonstrated significant reductions. These results indicate the potential of DIBH in mitigating RRHD, potentially lowering risks like coronary artery disease and myocarditis. Radiation pneumonitis (RP), an acute lung injury caused by radiation, poses a significant challenge in thoracic RT. RP occurrence, which is influenced by lung volume and radiation dose parameters, affects treatment plans and patient prognosis. V20 and lung Dmean are key predictors of RP, predominantly used in clinical practice. Limiting V20 to ≤ 30% reduces RP and preserves lung function [35]. Other predictors of RP include V5, V25, and V30, which indicate lung volumes exposed to specific radiation doses.
This meta-analysis revealed that the DIBH group demonstrated a lower ipsilateral lung Dmean than the FB group, but with poor evidence due to high heterogeneity. Similar results were observed for ipsilateral lung V5 and V10. However, ipsilateral lung V20 indicated a significant reduction in the Dmean for the DIBH group compared with the FB group, with low heterogeneity. During FB, the left lung’s proximity to the treatment field may cause higher radiation exposure. Chest and lung movements cause dose variations, potentially resulting in left lung overexposure. DIBH typically reduces the left lung Dmean compared with FB. The heart and left lung move away from the treatment field by requesting patients hold their breath in deep inspiration, thereby improving breast tissue targeting and reducing left lung radiation exposure. Most studies conducted with 3D-CRT or tangential IMRT have revealed that DIBH can reduce Dmean, V5, V10, and V20 in the left lung, although some have observed no significant difference [36–39]. Patient anatomy and geometry may play a dominant role in identifying the results. Among the contralateral lung doses, the DIBH group demonstrated a significantly lower Dmean than the FB group. The subgroup analysis of contralateral lung dose V5 revealed similar results, although V20 did not reach statistical significance.
Swamy et al. revealed that the heightened contralateral breast dose cannot be ignored for young patients (< 40 years old) in terms of the contralateral breast, as the risk of radiation-induced secondary cancers is increased for doses of > 1 Gy. An increased volume of the right lung and right breast was exposed to low doses in the VMAT plans. The low dose–volume was usually greater in VMAT due to multiple beam directions passing through regions outside the planning target volume. The issue of the volume of low doses in the contralateral breast and lung remains unknown for the potential risk of secondary cancers. In this meta-analysis, the forest plot indicated a significantly lower Dmean in the contralateral breast for the DIBH group than for the FB group because patients with DIBH showed increased contralateral lung volume and movement of the contralateral breast away from the treatment beams. This indicates a reduced risk of contralateral breast mastitis or the development of precancerous lesions. However, no statistically significant difference was found in contralateral breast V5.
A previous study indicated that tumor bed boost, arc angle, and lymph node involvement may influence the heart dose [21]. Our subgroup analysis revealed that all subgroups, i.e., breast only, with or without a tumor bed boost, and with or without nodal irradiation, who received VMAT treatment with DIBH showed reduced heart Dmean. This strategy is especially important for patients suffering from LSBC with a tumor bed boost or without nodal irradiation. DIBH is more effective than FB in reducing the heart Dmean.
Radiotherapy with the arc angle of > 180° (extensive partial arcs) demonstrated better heart Dmean reduction between the DIBH and FB groups than RT with the arc angle of < 180° (restricted partial arcs). Treatment plans using restricted partial arcs may reduce doses in the gantry incident direction compared with those with extensive partial arcs. However, restricted partial arcs may be necessary to maintain target coverage, compromising the benefits of few doses. Konstantinou et al. revealed low doses in the LAD and left ventricle Dmean in plans with four restricted partial arcs vs. two extensive partial arcs. Plans with four restricted partial arcs demonstrated slightly high heart Dmean, V20, and V40, but the differences were insignificant [40]. However, the results regarding which arc angle results in lower doses to OARs when combined with DIBH remain difficult to interpret. These subgroups may confound each other; for example, a different arc angle might be applied in the case of a tumor bed boost, thereby confounding the results. Alternatively, this meta-analysis had several limitations. Such anatomic difference described previously may affect the exposed dose of radiotherapy. The sample size was relatively small; the included studies were all case series (nonrandomized); and no follow-up plan existed. These factors could introduce biases and inaccuracies when applied in a clinical setting.